The present invention relates to an amplifying device and a radio.
Terrestrial digital broadcasting has a 13-segments structure. A service to receive one of 13 segments, i.e., a so-called one-segment broadcasting service is available to mobile terminals.
A one-segment broadcast signal may be transmitted from a long distance. In this case, a one-segment broadcast receiving unit in a cellular phone handset having a one-segment broadcast receiving function handles a very weak signal. A cellular communication signal which is transmitted from a transmitter in the immediate vicinity of the one-segment broadcast receiving unit and whose frequency is higher than that of a one-segment broadcast signal by several tens of MHz is very strong and serves as an interfering wave for the one-segment broadcast receiving unit.
Amplifying circuits and filtering circuits which handle a signal obtained by downconverting a one-segment broadcast signal are each often composed of a circuit using an operational amplifier. An operational amplifier has a parameter called a slew rate which represents a voltage variation (determined by signal amplitude×frequency) allowed per unit time.
In order to amplify and output a signal which is high in frequency and strength (which has a large amplitude) without distorting the signal, an operational amplifier needs to have a high slew rate. A slew rate is generally proportional to current consumption. It is thus necessary to increase current consumption in order to achieve a high slew rate.
A low slew rate suffices for a cellular phone handset having a one-segment broadcast receiving function as described above in the absence of an interfering wave (a cellular communication signal). However, a cellular phone handset needs to use an operational amplifier with a high slew rate in preparation for issuance of an interfering wave by the cellular phone handset. This causes a large current to be applied even when the slew rate of a signal is low and increases costs.